Charge Balance Techniques for Power Devices

ABSTRACT

A silicon wafer includes a silicon region of first conductivity type and a plurality of strips of second conductivity type pillars extending in parallel in the silicon region from a location along a perimeter of the silicon wafer to an opposing location along the perimeter of the silicon wafer. The plurality of strips of second conductivity type pillars extend to a predetermined depth within the silicon region.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a division of U.S. application Ser. No. 12/562,025,filed Sep. 17, 2009, which is a division of U.S. application Ser. No.11/396,239, filed Mar. 30, 2006, now U.S. Pat. No. 7,592,668, thecontents of which are incorporated herein by reference in their entiretyfor all purposes.

BACKGROUND

The present invention relates to semiconductor power device technology,and more particularly to charge balance techniques for semiconductorpower devices.

A vertical semiconductor power device has a structure in whichelectrodes are arranged on two opposite planes. When the vertical powerdevice is turned on, a drift current flows vertically in the device.When the vertical power device is turned off, due to a reverse biasvoltage applied to the device, depletion regions extending in thehorizontal and vertical directions are formed in the device. To obtain ahigh breakdown voltage, a drift layer disposed between the electrodes isformed of a material having high resistivity, and a thickness of thedrift layer is increased. This, however, leads to an increase in thedevice on-resistance Rdson, which in turn reduces conductivity and thedevice switching speed, thereby degrading the performance of the device.

To address this issue, charge balance power devices with a drift layercomprising vertically extending n regions (n pillar) and p regions (ppillar) arranged in an alternating manner has been proposed. FIG. 1A isa layout diagram of such a device 100. Device 100 includes an activearea 110 surrounded by a non-active perimeter region which includes a pring 120 and an outer termination region 130. The perimeter p ring 120has a rectangular shape with rounded corners. Termination region 130 mayinclude similarly shaped alternating p and n rings, depending on thedesign. Active area 110 includes alternately arranged p pillars 110P andn pillars 110N extending vertically in the form of strips andterminating along the top and bottom at the perimeter ring 120. Thephysical structure of the alternating p and n pillars in the active areacan be seen more clearly in FIG. 1B which shows a cross section view inarray region 110 along line A-A′ in FIG. 1A.

The power device depicted in FIG. 1B is a conventional planar gatevertical MOSFET with a drift layer 16 comprising alternating p pillars110P and n pillars 110N. Source metal 28 electrically contacts sourceregions 20 and well regions 18 along the top-side, and drain metal 14electrically contacts drain region 12 along the bottom-side of thedevice. When the device is turned on, a current path is formed throughthe alternating conductivity type drift layer 16. The dopingconcentration and physical dimensions of the n and p pillars aredesigned to obtain charge balance between adjacent pillars therebyensuring that drift layer 16 is fully depleted when the device is in theoff state.

Returning back to FIG. 1A, to achieve a high breakdown voltage, thequantity of n charges in the n pillars and the quantity of p charges inp pillars must be balanced in both the active area 110 and at theinterface between the active area and the non-active perimeter region.However, achieving charge balance at all interface regions, particularlyalong the top and bottom interface regions where the p and n pillarsterminate into perimeter ring 120, as well as in the corner regionswhere the n and p pillars have varying lengths, is difficult because ofthe change in geometry of the various regions. This is more clearlyillustrated in FIG. 1C which shows an enlarged view of the upper leftcorner of power device 100 in FIG. 1A.

In FIG. 1C, a unit cell in active area 110 is marked as S1. Active ppillar 111 (which is divided into a left half portion 111-1 and a righthalf portion 111-2) and active p pillar 113 (which is divided into lefthalf portion 113-1 and right half portion 113-2) are separated by an npillar 112. The sum (Qp1+Qp2) of the quantity of p charges Qp1 in theright half portion 111-2 of the active p pillar 111 and the quantity ofp charges Qp2 in the left half portion 113-1 of the active p pillar 113in unit cell 51 is equal to the quantity of n charges Qn1 in the activen pillar 112. An optimum breakdown voltage is thus achieved in all partsof active area 110 where such balance of charge is maintained.

As shown, the corner portion of the non-active perimeter region includesthe perimeter p ring 120 and termination region 130 with n ring 131 andp ring 132 which are arranged in an alternating manner. Perimeter p ring120 (which is divided into a lower half portion 121 and an upper halfportion 122) and termination region p ring 132 (which is divided intolower half portion 132-1 and upper half portion 132-2) are separated byn ring 131. The sum (Qpt1+Qpe) of the quantity of p charges Qpt1 in thelower half portion 132-1 of p ring 132 and the quantity of p charges Qpein the upper half portion 122 of ring 120 in unit cell S2 is equal tothe quantity of n charges Qnt in n ring 131. An optimum breakdownvoltage is thus achieved in all parts of the non-active perimeter regionwhere such balance of charge is maintained.

However, because of geometrical limitations, the quantity of p chargesand the quantity of n charges at the interface between the active areaand the non-active perimeter region are unbalanced in many places. Theabsence of charge balance in these regions results in a deterioration ofthe breakdown characteristics of the device. Thus, there is a need forcharge balance techniques which eliminate the prior art charge imbalanceproblems at the active area to non-active perimeter region interface,thereby leading to higher breakdown voltage ratings.

BRIEF SUMMARY

In accordance with an embodiment of the invention, a silicon waferincludes a silicon region of first conductivity type and a plurality ofstrips of second conductivity type pillars extending in parallel in thesilicon region from a location along a perimeter of the silicon wafer toan opposing location along the perimeter of the silicon wafer. Theplurality of strips of second conductivity type pillars extend to apredetermined depth within the silicon region.

In accordance with another embodiment of the invention, a silicon dieincludes a silicon region of first conductivity type and a plurality ofstrips of second conductivity type pillars extending in parallel in thesilicon region from one edge of the silicon die to an opposing edge ofthe silicon die. The plurality of strips of second conductivity typepillars extend to a predetermined depth within the silicon region.

In accordance with yet another embodiment of the invention, a method offorming a charge balance structure in a semiconductor die having asilicon region of first conductivity type includes forming a pluralityof strips of second conductivity type pillars extending in parallel inthe silicon region from one edge of the silicon die to an opposing edgeof the silicon die. The plurality of strips of second conductivity typepillars extend to a predetermined depth within the silicon region.

In one embodiment, the forming step includes forming a plurality oftrenches extending to the predetermined depth in the silicon region, thetrenches extending from the one edge of the silicon die to the opposingedge of the silicon die, and filling the plurality of trenches withsilicon material of the second conductivity type.

A further understanding of the nature and the advantages of theinvention disclosed herein may be realized by reference to the remainingportions of the specification and the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A shows a simplified layout diagram of a conventional chargebalance power device;

FIG. 1B shows a cross section view along A-A′ line in the power devicein FIG. 1C;

FIG. 1C shows an enlarged view of the upper left corner of the powerdevice in FIG. 1A;

FIG. 2 shows a simplified layout diagram for charge balance powerdevices in accordance with an exemplary embodiment of the invention;

FIG. 3 shows a simplified layout diagram for charge balance powerdevices in accordance with another exemplary embodiment of theinvention;

FIG. 4 shows a simplified layout diagram for charge balance powerdevices in accordance with yet another exemplary embodiment of theinvention; and

FIGS. 5 and 6 show simplified cross section views of the non-activeperimeter region wherein field plates are integrated with charge balancestructures according to two exemplary embodiments of the invention.

DETAILED DESCRIPTION

FIGS. 2-4 show simplified layout diagrams of dies wherein improvedcharge balance techniques are implemented in accordance with threeexemplary embodiments of the invention. These techniques advantageouslyeliminate the intricate design necessary to achieve charge balance atthe transition region between the active area and its surroundingnon-active perimeter region in prior art charge balance devices.

In FIG. 2, a die 200 housing a charge balance power device comprises anactive area 202 wherein many active cells are formed, and a non-activeperimeter region surrounding the active area. The non-active perimeterregion is defined by the distance from the horizontal edges of activearea 202 to corresponding edges of the die marked in FIG. 2 by letter X,and by the distance from the vertical edges of active area 202 tocorresponding edges of the die marked in FIG. 2 by letter Y. In general,the term “active area” is used herein to identify the region of thedevice in which active cells capable of conducting current are formed,and the term “non-active perimeter region” is used to identify theregion of the device in which non-conducting structures are formed.

Distances X and Y in FIGS. 2-4 are significantly exaggerated in order tomore clearly show the charge balance technique in these figures (inpractice, distances X and Y are significantly smaller than those shownin FIG. 2-4). Where the power device housed in die 200 is a MOSFET(e.g., similar to that in FIG. 1B), the boundary of active area markedin FIG. 2 by reference numeral 202 corresponds to the boundary of thewell region in which the MOSFET cells are formed.

As shown in FIG. 2, vertically extending p pillars 210P and n pillars210N are arranged in an alternating manner to thereby form a chargebalance structure. In one embodiment, active p pillars 210P are formedby creating trenches in the silicon and filling them with p-type siliconusing known techniques such as selective epitaxial growth (SEG). Ingeneral, the physical dimensions and doping concentration of the n and ppillars are optimized so as to obtain charge balance between adjacentpillars, similar to that described above in connection with unit cell S1in FIG. 1C.

In FIG. 2, unlike conventional charge balance devices wherein the p andn pillars in the active area terminate at the boundary of the activearea, the active p and n pillars extend through both the active area andthe non-active perimeter region, as shown. This eliminates the chargebalance concerns at the edges and corners of the active area, thusachieving perfect charge balance and breakdown characteristics whilesignificantly simplifying the design of the device.

In one embodiment, distances X and Y are chosen to ensure full depletionoutside the active area. In one embodiment wherein p pillars are formedby forming trenches in silicon, each of distances X and Y is equal to orgreater than a depth of the p pillar trenches. While the vertical edgesof active area 202 are shown in FIG. 2 to fall within n pillars, theactive area could be expanded or contracted so that the vertical edgesof the active area fall within p pillars. As such, there are nomisalignment issues with respect to the edges of active area 202 and thepillars. In one embodiment, the starting wafer may include the p and npillars as shown in FIG. 2, and the power device including its activearea and other regions are formed using known manufacturing techniques.

FIG. 3 shows another embodiment which is similar to that in FIG. 2except a discontinuity is formed in the vertically extending p pillarsin each of the upper and lower non-active perimeter region. Thediscontinuities form a horizontally extending n strip 320N which breaksup each p pillar into two portions 310P-1 and 310P-2 as shown in thelower non-active perimeter region. The discontinuity in the p pillarsdisturbs the fields in the non-active perimeter region so as to reducethe fields along the silicon surface in this region. This helps improvethe breakdown voltage in the non-active perimeter region.

In one embodiment, a spacing B from the edge of active area 302 to nstrip 320N is determined based on the voltage rating of the powerdevice, photo tool limitations, and other performance and design goals.In one embodiment, a smaller spacing B is used enabling finer fielddistribution adjustments. Once again, the dimensions in the non-activeperimeter region (X, Y, B) are all exaggerated to more easily illustratethe various features of the invention.

FIG. 4 shows a variation of the FIG. 3 embodiment wherein multiplediscontinuities are formed in each p pillar in each of the upper andlower non-active perimeter regions, thus forming multiple n strips 420N,430N in these regions. Multiple discontinuities enable higher voltageratings. As shown, outer strip 430N is wider than inner strip 420N. Theconsiderations in selecting the widths of the N strips and the spacingtherebetween are similar to those for conventional termination guardrings. In one embodiment, the n strips in FIGS. 3 and 4 are formed asfollows. During the process of forming the p pillars, a mask is used toprevent formation of p pillars at the gap locations along the p pillars.

The techniques in FIGS. 2-4 may be combined with other edge terminationtechniques as needed. In particular, termination field plate techniquesmay be advantageously combined with the embodiments in FIGS. 2-4 tofurther reduce the fields at the silicon surfaces in the non-activeperimeter region. Two examples of such combination are shown in FIGS. 5and 6.

FIG. 5 shows a cross section view along a region of the die at an edgeof the active area. In FIG. 5, the active area extends to the left ofp-well 502, and the non-active perimeter region extends to the right ofp-well 502. As in FIGS. 2-4 embodiment, p-pillars 510P and n-pillar 510Nextend through both the active area and non-active perimeter region. Asshown, p-pillars 510P terminate at a depth within N-epitaxial layer 512,and those portions of N-epitaxial layer 512 extending between p-pillars510P form the n-pillars 510N of the charge balance structure. Floatingp-type diffusion rings 504A-504C are formed in the non-active perimeterregion and extend around the active region. As can be seen, the spacingbetween adjacent rings progressively increases in the direction awayfrom the active region. A dielectric layer 506 insulates rings 504A-504Cfrom overlying structures (not shown). P-well 502 may either be the lastp-well of the active area or form part of the termination structure. Ineither case, p-well 502 would be electrically connected to the activep-well.

FIG. 6, similar to FIG. 5, shows a cross section view of a region of thedie at an edge of the active area, with the active area extending to theleft of p-well 602 and the termination region extending to the right ofp-well 502. P-pillars 610P and n-pillar 610N extend through both theactive and termination regions. As in the FIG. 5 embodiment, p-pillars610P terminate at a depth within N-epitaxial layer 612, and thoseportions of N-epitaxial layer 612 extending between p-pillars 610P formthe n-pillars 610N of the charge balance structure. In this embodimenthowever, a planar field plate structure is formed over the non-activeperimeter region. The planar field plate structure includes apolysilicon layer 608 extending over the non-active perimeter region,and a metal contact layer 614 electrically connects polysilicon layer608 to p-well 602. A dielectric layer 606 insulates the charge balancestructure in the non-active perimeter region from the overlyingpolysilicon layer 608 and other structures not shown. As in the FIG. 5embodiment, p-well 602 may either be the last p-well of the active areaor form part of the termination structure. In either case, p-well 502would be electrically connected to the active p-well.

While FIGS. 5 and 6 show two different edge termination techniques,these two techniques may be combined in a variety of ways. For example,in an alternate implementation of the FIG. 6 embodiment, a number offloating p-type diffusion rings are included in the non-active perimeterregion in similar manner to that in FIG. 5 except that the p-typediffusion rings are placed to the left of field plate 608. As anotherexample, in an alternate implementation of the FIG. 5 embodiment, aseparate planar field plate is connected to each floating p-typediffusion ring 504A-504C.

The various charge balance techniques disclosed herein may be integratedwith the vertical planar gate MOSFET cell structure shown in FIG. 1B,and other charge balance MOSFET varieties such as trench gate orshielded gate structures, as well as other charge balance power devicessuch as IGBTs, bipolar transistors, diodes and schottky devices. Forexample, the various embodiments of the present invention may beintegrated with any of the devices shown for example, in FIGS. 14,21-24, 28A-28D, 29A-29C, 61A, 62A, 62B, 63A of the above-referenced U.S.patent application Ser. No. 11/026,276, filed Dec. 29, 2004 whichdisclosure is incorporated herein by reference in its entirety for allpurposes.

While the above provides a detailed description of various embodimentsof the invention, many alternatives, modifications, and equivalents arepossible. Also, it is to be understood that all numerical examples andmaterial types provided herein to describe various embodiments are forillustrative purposes only and not intended to be limiting. For example,the polarity of various regions in the above-described embodiments canbe reversed to obtain opposite type devices. For this and other reasons,therefore, the above description should not be taken as limiting thescope of the invention as defined by the claims.

1-18. (canceled)
 19. A silicon wafer comprising: a silicon region offirst conductivity type; and a plurality of strips of secondconductivity type pillars extending in parallel in the silicon regionfrom a location along a perimeter of the silicon wafer to an opposinglocation along the perimeter of the silicon wafer, the plurality ofstrips of second conductivity type pillars extending to a predetermineddepth within the silicon region.
 20. The silicon wafer of claim 19wherein the first conductivity type is n type and second conductivitytype is p type.
 21. A silicon die comprising: a silicon region of firstconductivity type; and a plurality of strips of second conductivity typepillars extending in parallel in the silicon region from one edge of thesilicon die to an opposing edge of the silicon die, the plurality ofstrips of second conductivity type pillars extending to a predetermineddepth within the silicon region.
 22. The silicon wafer of claim 21wherein the first conductivity type is n type and second conductivitytype is p type.
 23. A method of forming a charge balance structure in asemiconductor die having a silicon region of first conductivity type,the method comprising: forming a plurality of strips of secondconductivity type pillars extending in parallel in the silicon regionfrom one edge of the silicon die to an opposing edge of the silicon die,the plurality of strips of second conductivity type pillars extending toa predetermined depth within the silicon region.
 24. The silicon waferof claim 23 wherein the forming step comprises: forming a plurality oftrenches extending to the predetermined depth in the silicon region, thetrenches extending from the one edge of the silicon die to the opposingedge of the silicon die; and filling the plurality of trenches withsilicon material of the second conductivity type.
 25. The method ofclaim 23 wherein the first conductivity type is n type and secondconductivity type is p type.